Control of alternating current motors



Dec. 7, 1948. R. B. HUNTER 2,455,491

CONTROL OF ALTERNATING CURRENT MOTORS Filed April 28, 1944 Q 2 Sheets-Sheet 1 Dec. 7, 1948. R. B. HUNTER 2,455,491

CONTROL OF ALTERNATING CURRENT MOTORS Filed April 28, 1944 2 Sheets-Sheet 2 Patented Dec. 7, 1948 CONTROL OF ALTERNATIN G CURRENT MOTORS Richard B. Hunter, Shorewood, Wis., assignor to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application April 28, 1944, Serial No. 533,147 14 Claims. 318-4203) This invention relates to improvements in control of alternating current motors and although not limited thereto is particularly advantageous in control of such motors where employed for hoisting and lowering widely varying loads.

In the application of R. P. Anderson, Serial No.

' 482,273, filed April 8, 1943, now Patent 2,436,413,

issued February 24, 1948, and assigned to the assignee of the instant application there is disclosed a very satisfactory control for motors employed in the aforementioned service, which control through the medium of an adjustable autotransformer unbalances the motor primary and ailords multi-speed lowering comparable to that obtainable in a direct current system. However, an adjustable auto-transformer requires protection against short-circuiting of the sections thereof in commutating its connections and hence is better suited to the electromagnetic type of control than to mechanical types including the drum type, whereas the drum type of control is frequently preferred. Of course by special design of the control drum safe control of the autotransformer may be afforded, but the drum then becomes complicated and costly.

The present invention has among its objects to afiord lowering control having substantially the same speed torque characteristics as where the aforementioned adjustable auto-transformer is employed, but Without resort to circuit commutation of the auto-transformer and through the medium of means which may be controlled by a drum of relatively simple construction and low cost.

Another object is to provide a system equally well suited to the electromagnetic type of control means and obviating need of the special provisions required where adjustment of the autotransformer is necessary.

Various other objects and advantages of the invention will hereinafter appear.

According to the present invention it is proposed to employ an auto-transformer for unbalancing the motor primary but instead of adjusting the auto-transformer to obtain the desired speed torque curves it is proposed to vary the speed torque characteristics through impedance control of the motor primary. The possibility of using primary resistance alone without resort to an auto-transformer is known, but the combination of auto-transformer and impedance herein disclosed has advantages over control through resistance alone as well as over control by an auto-transformer alone. This combination affords for a given lowering speed a higher torque than has been obtained by control through resistance alone, whereas advantages-thereof over the adjustable transformer have been hereinbefore mentioned.

The accompanying drawings illustrate an embodiment of the invention which will now be described, it being understood that the embodiment illustrated is susceptible of various modifications without departing from the scope of the appended claims. In the drawings,

Figure 1 is a diagrammatic showing of a motor and a complete controller therefor of the drum type;

Figs. 2 to 7 show diagrammatically in simplified form circuit commutatio-ns of the motor which are afforded by the controller of Fig. 1;

Figs. 2 to 7 are vector showings for the circuit commutations of Figs. 2 to '7, respectively, these vectors being for zero speed and not necessarily quantitative, and

Fig. 8 depicts approximately only, the speed torque characteristics obtainable through the circuit cornmutations illustrated in Figs. 2 to '7.

Referring to Fig. l, the same illustrates a slip ring motor l0 having a stator to be supplied from supply lines L L and L the stator having terminals T T and T The rotor of the motor has terminals M M and M and said rotor has for control thereof in a known manner series resistors for its several phases, namely resistors R to R R to R and R to R Also as is customary the motor has associated therewith a brake having a shunt winding, its terminal B thereof connected to motor terminal T and with its terminal B connected to motor terminal T The connections between the stator of motor I0 and the supply lines include a triple pole switch H preferably of the electromagnetic type to be controlled as by drum contacts not shown or in any other preferred manner. One pole of the switch ll afiords a direct connection from line L to the motor terminal T whereas the connections between the motor and lines L and L must be made through the drum type controller l2.

The drum controller I2 is of conventional form comprising two sets of contact segments and an intermediate set of contact fingers. As will be understood, when the segments stand in that relation to the contact fingers depicted in the drawing the drum is in off position from which it is movable in one direction to engage with certain of the fingers, the hoisting segments or alternatively in a reverse direction to engage with certain of said fingers the lowering segments.

The hoisting segments are arranged to accomplish hoisting control in a well known manner and accordingly this portion of the control will not be described in detail. For simplicity of illustration, Fig. 1 shows the circuit connections between the motor and the drum in cable form, but the drawing employs customary reference characters enabling all circuits to he readily traced. Thus for example it will be understood that contact finger T has a direct connection with motor terminal '1, whereas contact finger R has direct connection with common point B of resistors E -R and PA -R The lowering segments of the drum comprise interconnected segments l3 andld which engage and bridge contact fingers 8 and L in all of the six lowering positions of the drum. Alsoa segment it engages contact finger T in all lowering positions of the drum, this segment being'interconnected with segments iii, W, l8, l9 and 20 which progressively engage contact fingers PR PR "PR PR and Segments iii and VI engage their respective fingers only in the first and second positions, respectively, of the drum. Segment Iii engages its respective finger in the thirdand fourth positions of the drum, while'segm'ent f9 engages its respective finger in fourth andfifth 'positions'of the'druin, segment Ellengagin its respective finger only in the sixth po sition of the drum. Additionally the lowering section of the drum comprises interconnected segments 2E, '22 and to respectively engage contact fingers R E and R in the first four lowering positions of the drum, and a pair 24 of interconnected segments to bridge contact fingers R and RA in the fourth, fifth and sixth positions of the drum.

The drum contact segments'and fingers above described with the exception of segments ii to 24 and their respective fingers are provided for control of the motor primary through the medium of an 'auto-transforniei"zli and impedances P'R PR 'PR -PR and PR -PR which it will be assumed comprise resistors although they might comprise inductors or capacitors. The cirsuits for the transformer and the resistances just'menti'oned are shown. in simplified form in Figs- 2 to '1.

Priorto considering the primary connections it is to be noted that inthe first four lowering positions of the drum all secondary resistance is included in circuit except resistors R -R "d -R and Po -R which are short-circuited by the-bridging of contact fingers R R and R by the segments M, and 23. @n the other hand, the resistors just mentioned are included in-the motor secondary circuit in the fifth and sixth lowering positionsof the drum as the result of disengagement of segments El, 22' and 23 from their respective contact fingers, and engagement ofsegments'M; with their contact fingers to connect terminal R of resistor R -R to the common' point of the resistors of the other phases.

Referringto Fig. 2, it shows the circuit connections forthe motor primary as established in the first lowering position of the drum, assuming the line switch i l to be closed. Considering these connections it will be observed that the motor terminal T has a direct connection to line L through one pole of the switch l 5, while a second Role Of switch 3 i connects to line L the lower terminal i of the auto-transformer winding 25, the upper terminal of said winding being connected to motor terminal T by engagement of drurn contact segments l5 and ltlwith their cooperating c'ontact'fingers T and PR in the first and such connections of the motor are like those afforded in the controller disclosed in the earlier application aforementioned with consequent over-voltage unbalancing of the motor primary as depicted in the vector showing of Fig. 2 and withsp'eed torque characteristics as depicted by curve 1 ofFig. 8. As depicted by curve i of Fig. 8, the motor is at zero speed capable of exert-- ing some hoisting torque, but this torque is too small to prevail over the weight of an empty hook and friction.

"As will'be apparent from Fig. l, the aforedesoribed connections between line L and the transformer winding, also between line L and the transformer, and between the transformer and motor terminal T remain the same for all lowering positions of the drum, assuming the ma'inswitch H to remain closed.

Referring-to Fig. 3, it shows the motor primary connections established in the second lowering p'ositionof the drum, the change effected being introduction of resistor FR -43R into the con-- nection between the transformer and motor terminal T This'is effected by engagement of the drum contactfingers iii and ill with their respective fingers to bridge the same and by disengagement of contact segment is from its contact 'firiger. This change in connections effects the vector change shown in Fig. 3 phase I now leading phase 3 instead of lagging phase 3 as in Fig. 2 The speed torque characteristic of the mot'orthenbecomes that depicted by curve 2 of FigJ-S, the motor then being capable at zero speed of "exerting a small lowering torque.

lteferring to Fig. it shows the primary connections established with the drum in its third lowering position, the change effected being that of inclusion of 'a second step of resistance PR PR between the transformer and the motor terminal T This change is effected by engaging contact segments in and it with their respective fingers T and PR to bridge the same and by disengaging segment ill from its respective finger to interrupt the direct connection between resistor PR -PR and the motor terminal T This change in connections efiects the vector chan'ge'shown in Fig. 4 and the change in speed torque characteristic depicted by curve 3 "of Fig. '8 which depicts an increased lowering torque.

Referring to Fig. 5, it shows the primary connections established in the fourth lowering position of thg'drum, the change effected being that of connecting motor terminal T to line L through resistor PR -PR while preserving the connections 'of Fig. 4:. This change in connectio'ns is eifected by the segments iii and i9 bridgin'gcontact fingers T and PR and it effects the vector change shown in Fig. 5 and produces the speed torque characteristics depicted by curve "of Fig.8,' with an increased lowering torque.

Referring to Fig. 6, it shows the primary con nections established with the drum in its fifth lowering position, the change in connections beingthat-eii'ected by'disengagement of segment i'8 fromits respective contact finger to interrupt the connection between motor terminal T and the transformer. This results in the vector change depicted in Fig. 6 and the change in speed torque characteristic depicted by curve 5 of Fig. 8, with a further increase in lowering torque.

Referring to Fig. 7, the same shows the circuit connections established with the drum in its'sixth lowering position, the change effected comprising direct connection of motor terminal '1 with line L by engagement of segments l5 and 20 with their respective contact fingers and by disengagement of segment [9 from its contact finger. This change effects the vector change depicted in Fig. '7 with a consequent change in speed torque characteristic, as depicted by curve 6 of Fig. 8.

As will be apparent from the curves of Fig. 8, the control thus afiorded thus very closely approximates for both light and heavy loads the control afforded by the aforementioned adjustable transformer, with a torque for any given speed which is relatively high although slightly lower than that obtainable with the aforemen tioned adjustable auto-transformer type of control.

As will also be apparent, the control illustrated involves the problem of shifting the connection of terminal T from the auto-transformer over to line L for certain results while always having some connection for said motor terminal, and as hereinbefore indicated this problem may be met through use of impedances other than of the resistor type.

What I claim as new and desire to secure by Letters Patent is:

1. In a braking system for an alternating current motor, the combination with means comprising an auto-transformer to supply power to the motor primary with voltage unbalance of the motor primary for high brakin action of the motor, of impedance and means to include said impedance in circuit with the motor primary while under the influence of said auto-transformer, for reduced braking action of the motor.

2. In a braking system for an alternating current motor, the combination with means comprising an auto-transformer to supply power to the motor primary with voltage unbalance of the motor primary for high braking action of the motor characterized by some torque of the motor at zero speed, of impedance and means to include said impedance in circuit with the motor primary while under the influence of said auto-transformer, for reduced braking action of the motor characterized by a reversed torque of the motor at zero speed.

3. In a braking system for an alternating current motor, the combination with means comprising an auto-transformer to supply power to the motor primary with voltage unbalance of the motor primary for high braking action of the motor, of impedance and means to subject the motor primary while under the influence of said auto-transformer to influence by said impedance to reduce the braking action of the motor to difierent degrees selectively.

4. In a braking system for an alternating current motor, the combination with means comprising an auto-transformer to supply power to the motor primary with voltage unbalance of the motor primary for high braking action of the motor, of impedance and means to subject the motor primary while under the influence of said auto-transformer to influence by said impedance to reduce the braking action of the motor to different degrees selectively, each step of reduced braking action being characterized by torque of the motor at zero speed for kick-off.

5. In a braking system for an alternatin current motor, the combination with means comprising an auto-transformer to supply power to the motor primary with voltage unbalance of the motor primary for high braking action of the motor, of impedance and means to subject the motor primary while under the influence of said auto-transformer to influence by said impedance to reduce the braking action of the motor to difierent degrees selectively, all such braking actions of the motor being characterized by some torque of the motor at zero speed, the torque for the high braking action being the reverse of that for the reducing braking actions, and the zero speed torque for reduced braking actions being graduated according to the reduced braking action selected.

6. In combination, a polyphase supply circuit, a polyphase motor, means including autotransformer to establish for motor braking action, connections between the motor primary and said supply circuit and to effect unbalance of the motor primary for high braking action of the motor, and means comprising impedance first to increase the impedance between the auto transformer and one terminal of the motor, then to establish an impedance connection between said motor terminal and one line of said supply circuit and then to disconnect said motor terminal from said auto-transformer, the last mentioned means affording through the medium of such circuit changes graduated reduced braking actions of said motor.

7. In combination, a polyphase supply circuit, a polyphase motor, means including an autotransformer to establish for motor braking action, connections between the motor primary and said supply circuit and to efie-ct unbalance of the motor primary for high braking action of the motor, and means comprising impedance first to increase the impedance between the autotransformer and one terminal of the motor, then to establish an impedance connection between said motor terminal and one line of said supply circuit and then to disconnect said motor terminal from said auto-transformer, the last mentioned means aifording through the medium of such circuit changes graduated reduced braking actions of said motor, each step of reduced braking action being characterized by torque of the motor at zero speed for kick-off.

8. In combination, a polyphase supply circuit, polyphase motor, an auto-transformer, means to establish interconnections between said transformer and the motor primary and line connections for both the motor primary and said autotransformer for high braking action of said motor through unbalancing of the motor primary, and means comprising impedance for first increasing the impedance between an end terminal of the auto-transformer and one terminal of the motor primary for a reduced braking action of said motor, then establishing an impedance connection between said motor terminal and one line of the supply circuit for further reduction of the braking action of said motor, then interrupting the connection between said motor terminal and said end terminal of said auto-transformer for further reduction of the braking action of said motor and then reducing the impedance of the line conaesaaor b. nection of said motor terminal for further reduc tion of. the braking action of said motor.

9. In combination, a polyphase supply circuit, a polyphase motor, an auto-transior1ner, means to establish interconnections between said trans iormer and the motor primary and line connections for both the motor primary and said autotransformer for high braking action of said motor through unbalancing oi the motor primary, and means comprising impedance for first increasing the impedance between an end terminal of the auto-transformer and one terminal of the motor primary for a reduced. braking action or" said motor, then establishing an impedance connection between said motor terminal and one line of the supply circuit for further reduction of the braking action of said motor, then interrupting the connection between said motor terminal and said end terminal of auto-transformer for further reduction of the braking action of said motor and then reducing the impedance of the line connection of said motor terminal for further reduction of the braking action of said motor, all of braking thus provided being characterizcdby motor torque at zero speed, the motor torque for the highbralring step being the reverse of that for all steps of reduced braking.

it In combination, a polyphase supply circuit, a polyphase motor, a single phase auto-transformer, means to establish for high braking action of the motor, connection of its primary to said.auto-transformer and connection of both to said supply circuit with said auto-transformer across two phases and with an overhanging section of said auto-transformer connected to a terminal or the motor primary, and-means comprising inpedance to effect graduated reduction of the braking action of the motor by first introducing impedance in the connection between said auto-transformer and said motor terminal, then connecting said motor terminal through in1pedance to a line of said. supply circuit, and then disconnecting motor terminal. from said auto-transformer.

ii. A method of reducing the braking action of an alternating current motor having its primary supplied. through an autmtransformer for unbalance of the motor primary and for high braking action characterized by some motor torque at zero speed, which comprises the step of including.

impedance in certain of the connections between the auto-transiormer and the motor for a re duced braking action characterized by a reversed motor torque at zero speed.

1.2. A method of reducing the braking action of an alternating current motor having its primary supplied through an auto-transformer for unbalance of themotor'primary andfor highbraking;-v action characterized by somemotor torque atizero speed; whichcomprises the" step of includingim-;-

pedance in certain-of the connections between the.

auto-transformer and the motor for a reduced. braking action characterized by a reversed motor torque at zero speed, and which comprisesfor further reduction of braking action the step of connecting to one line of the supply circuit through an impedance a terminal of the motor. primary having the aforementioned impedance connection withthe auto-transformer;

13: A method of reducing the braking action of: an alternating current motor having its primarysupplied. through an auto-transformer for unbalance of. the motor primary and for high braking action characterized by some motor torque at zero speed, which-comprises the step of including impedance in certain of the connections between the auto-transformer. and the. motor' for a reduced'braking action characterized by a reversed motor torque at zero speed and which comprises for further reduction or braking action the step of connecting to one line of the supply: circuit; through an impedance a terminal of the motorprimary having the aforementioned impedance" connection with the auto-transformer and the step or thereafter disconnecting such motor" terminal from, the auto-transformer.

14". A method of effecting varying braking action of an alternatingcurrent'motorhaving its, primary supplied by an auto transformer for braking which comprises the step-of connecting; the motor primary to the auto-transformer. for braking action characterized by amotor torque in a given direction at zero motor speedandthe step of'varying the impedance of certain of the motor primary connections relative to other of? the same thereby to produce without interruption of the power connections or the motor primary, 3, reverse motor torque at zero motor speed;

RICHARD B. HUNT-ER REFERENCES CITED The following'references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 913,415 Mershon Feb.- 23, 1909 2,228,078 Gulliksen' Jan. 7, 1941.

FOREIGN PATENTS Number Country Date 212,698 Switzerland Mar. 3,1941. 570,284 Germany Jan. .26, 1933' 

